Updated JSQMessagesViewController to Version 6+

Same functionality as previously but with the updated JSQMessagesViewController.

![JSQMessagesViewController Version 6](https://s3.amazonaws.com/f.cl.ly/items/270B1o300H281X3T1e1m/iOS_Simulator_Screen_Shot_Dec_17__2014__5_13_41_PM.png)
@davidchiles A comment here mentioning that setting this value does a comparison of against the previous value to  invalidate the OTRImages cache. If you use the `/** comment */` style instead of `// comment` it will show up in the auto-complete UI. 

```obj-c
/** 
 * Setting this value does a comparison of against the previously value
 * to invalidate the OTRImages cache. 
 */
@property (nonatomic, strong) NSData *avatarData;
```

Fortunately NSCache claims to be thread-safe, which makes the internal side-effects less of a problem. Having global side-effects in your setters and getters of models like this can cause weird problems and should generally be avoided and moved to the consumer if possible. In this case it seems convenient enough to leave in the model though.